The United Nations mission in Congo will pursue the Allied Democratic Forces rebels who killed 14 peacekeepers earlier this month in the single deadliest attack on a peacekeeping mission in nearly 25 years, the U.N. said Tuesday.

The Dec. 7 attack on a peacekeeping base also killed at least five Congolese soldiers. U.N. spokesman Stephane Dujarric said the U.N.'s official death toll is 14 peacekeepers dead and one missing, which has accounted for a shifting death toll.

In a new indication of the severity of the hours-long attack, Jean-Pierre Lacroix said the peacekeeping base has been "recovered and reoccupied." It also is being rebuilt and reinforced, he said on Twitter.

In a visit to the region, Lacroix told peacekeepers that the U.N. will protect them and seek justice.

"We have spoken with the Congolese government and we will reinforce our military presence and weapons in the Semuliki region to track down and neutralize the negative groups in eastern Congo," he said. "We know why ADF attacked us, because we disrupted their activity."

The peacekeeping base is about 45 kilometers (27 miles) from the town of Beni, which has been repeatedly attacked by the ADF rebels. The base is home to the U.N. mission's rapid intervention force, which has a rare mandate to go on the offensive against armed groups that number in the dozens in the vast, mineral-rich region.

Uganda and Congo launched a joint military operation last week against the ADF rebels. Human Rights Watch says the group has killed more than 1,000 people in eastern Congo since October 2014.